tv [untitled] October 16, 2011 3:30pm-4:00pm PDT
opposed to individual footings. eventually, as you move closer and closer to the shoreline, where you cannot get economically justified basement, we use piles. those are the most expensive, where we drive -- the old days, again, we used to take trees, turn them upside down and drive them until they stop. with concrete, we went to circular piles, then square piles. now we have all overcast and micro piles, numerous file types that can be tailored through the actual conditions that exist at the site. this particular case, they're making an excavation in the rock. there are some parts of san francisco where the rock is shallow.
they go down and grab the rocked, and they put them at foundation on the rock and they can build 60-story tower. perfect example, as you approach the bay bridge, on the left there is the building called 1 rincon hill. that is 63 stories tall with a basement, and it sits on a foundation in rock. if you look to the left, you see the infinity. it has the shaped. that is the rock site. they dug down, put it on rock. as you move north, you come to the southern pacific building or the embarcadero centers or the trans america towers. there, the rock is 40, 60 feet deep. but the decision was made that we need parking.
so they elected to dig down to the dents supporting soil and build a foundation on the dance material below the bay mud. so that is a rock site, rock expedite -- rock excavation. this is part of the temporary shoring system that retains the walls of the excavation and prevents the building, other buildings from falling into the hole. >> where were we? >> we were driving around in the marina. we were looking over the perfectly flat marina green, at the whole marine area, which is also perfectly flat because it was artificial filled from the 1915 exposition. >> that is correct.
it was hydraulics and pumped in from the bay. same principle, they built a dike on the outside it, and then pumped in the sand. and as we all know, in 1989, a lot of damage. lawrence can speak to the concept of soft story problems with the buildings, and the look of viable soil. in perfect concert with each other, you get major damage, sometimes collapsed. that is the subject of -- i guess we did a video of that about a year ago? >> would try to pick up all the hazards. >> this is recreation of the natural landscape? >> yes, but the four points set
on a rock. there is a natural rock outcrop their. the natural material was reclaimed land, because it was the original airport for san francisco. it was about 4,000 feet long. in those days, they had small propeller planes and it was adequate. the bridge itself is supported on case sounds that go to rock. -- is supported on casings that go to rock. it is predominantly sandstone and shale. it was the shape of a dome. it was dome shaped. because san francisco was growing, because they needed the dike, that allowed miners to come in and open quarries.
and they blasted. they had no concern about environmental issues, no concern about how they would leave the quality of the rock. as a result, they opened up fractures in the rocke. even today, we are paying for their mining negligence. we read it just last year where 455 vallejo, there was a rockslide that affected properties on broadway and montgomery. at rock slide occurred because of blasting that occurred back in the turn-of-the-century that left the rock week. 100 years later, the weak rock cannot stand there anymore and it fell. this slide is a long sansome street, where the city chose to protect sansome street and the cars, and they built this
catchment wall. the sole purpose of this wall is to catch rocked that falls from private property. >> and every year or two, we get a call saying that rocks have fallen down the road and spilled over. this is the famous slide where we had a building at the top fall down this hill. this is the 22 street building. this was a watercolor done in 1945 by david mandel which -- by david mandelowiz. this building started gradually falling and sliding down. >> they had to replace the street right there. and in the process of jack hammering out the old st., they weakened the rocked. then the rockfall occurred. it undermined the building.
once the building was undermined, it could no longer state occupied, and the decision was made to remove the building. as you recall, it made headlines. eventually, the building was removed and a new wall was built. the lot where 22 once sat is still there. there are big disputes over whether it can be developed safely predict -- developed safely. and the filbert street stairs, because of the rockfall that occurred, new stairs were built, and they were built in a manner that when rockfalls occur, that actually lift out the stairs and put them on the ground and allow the crane to get back there to access the rockfall. >> beyond that, the building that we see at the bottom of the
rockfall was actually designed to allow barack to fall off and be compounded by the building -- actually designed to allow barack to fall off and be impounded by the building. >> that is the corner of sansome and green. every couple years, there is a rock fall that occurs behind that building. >> now we are on russian hill. this is the most beautiful location of exposed bedrock. across the street from the art institute on chestnut. >> that and tires sitting -- that entire setting, that is a beautiful setting, the deck and the panorama of the city. >> cannot be it. now we are on at leavenworth street, around the corner of chestnut or francisco. >> these walls are common
throughout the city, and they were built as part of the stimulus package following the great depression. it is known as the noriega wall, the leavenworth wall. they were well engineered. at that time, because of the depression, they really gave 100% to the work they were doing. even today, these walls, there is very little movement in these walls, they are performing well. you can check them against earthquake loads, there are more than adequate to resist any earthquake load that you can throw at them. i applaud the results of that stimulus package. >> this particular slide illustrates a bunch of things. one, the hill seemed beyond going down after the lower
roadway, goes down quite steeply to pastry -- to bay street and soon thereafter hits the point where there is artificial fill. so you can see dramatically the naturally occurring landforms and the filled terrace. this was also interesting in that you can see alcatraz and angel island at beyond, which of course are the natural peaks of the underground mountains in the bay. so this slide wraps up a lot of conditions. >> is this back on the docks? >> no, these walls were designed as gravity walls. they had the 1 inch steel rod at
the top, and a 1 inch rod that went from the wall to the middle of the street. they had a big blob of concrete that went from the wall, the rod, and then the block of concrete. if the wall tried to move, it would engage the rod, which would engage the block of concrete. these were built as gravity walls. they're very, very wide at the base. gosh, some of them are six, eight, 10 feet wide, and they are like dams. they are faced with basalt blocks that came from the quarries in marin county. they have performed very well. the classic gravity wall. there is enough friction along the base, and there is a slope on the backside, so there is enough weight pushing down on
the wall that keeps it from moving. >> this is on broadway, along rage and russian hill. >> that is a completely different design. this is truly what is called a cantilever retaining wall. it is designed to rotate and move. you can see the quality of construction. probably what they did in those days, they would take the rock that was present in the vicinity and mix it with the cement and create concrete. so there is no quality control. the strength of that wall various props -- varies from batch to batch, but it is safe. that is a gatt -- that is a gravity wall with a buttress in it. the diagonal strokes are buttressedes to keep the wall
from moving. the wall is actually providing support to the house is above. you cannot tolerate any movement. so they put these angles of concrete called buttresses. >> what would be under that? >> that is the cathedral at the corner of california and taylor. the rock under here is franciscan. it is sandstone. it is probably the hardest rock in san francisco at this location. we have just recently completed an investigation for the expansion of cathedral school, and they chose to go down 20 feet below the surface of the street, and it was incredibly hard. it was harder than the concrete that was placed as a foundation. the rock beneath the concrete
was 10 times stronger than the concrete that is used to support the building. >> okay, moving to another part of the city, this is the eastern side. >> this is on army street? >> yes, this is army st., se. look at all that stuff. it is an active city. >> a lot of the old industrial, the american can co., goodman lumber. all that good stuff. >> wow. >> and this building is one of the examples of remaining 1906 earthquake damage. it has been repaired above, and that is where they repaired the damage. what did they found these buildings on, back in the early
days? >> those days, remember i mentioned early on, it would use of redwood grillage and they would extend the grillage up far enough so it would spread out the load. today, one would know when to evaluate a building like this, we say to ourselves, there is no way this building can be standing. the bearing pressures that are being posed on the soil far exceed the strength of the materials present, yet it works. it could be such a phenomenon as arching and other things to keep the building standing. typically, today, we would support this building on pilings. we would support this going down to more confident material. within this is phil, bay mud, shall mud, and then down sand. re tom hill, at one time second
street ended at howard. at that point, there was a large hill. the wealthy people of san francisco lived on top of the hill, and churches were built along howard. then the city fathers in the 1920's let us knock down the hill, let us extend second street, and it finishes at the ballpark now. as you move south along second, that whole area, you get into rock. many of these old warehouse buildings, as you approach rincon hill, are sitting on rock. if you go down to king street, which is the street that fronts the ballpark, if you cross the street from the ballpark and the look on the sidewalk, there is actually a little brass plates
that depict the location of the bluff that existed, the transition from the beach to the vertical cliffs that existed at second street. at second and king. all of that was taken down, and they have built warehouses. now at his condominium developments. and portions of mission bay. the first building of mission bay, third and townsend -- i am sorry, third and king, it is a rock site. if you go further into mission bay, it goes from rock to week bay mud. -- weak bay mud. this is on the folsom? >> the old wonder bread bakery. a little bit higher up.
>> that particular building, if you look closely, the rock has actually receded back because of rockfalls. it was too expensive to put in foundations, so the foundation for that building is right on the surface. over the years, the rock face has receded to the point now where the flooding in the face of the bluff come together -- the footing and the face of the building and the bloc come together and are exposed. >> the confidence in the building is starting to be undermined. down at the bottom of the hill, out towards the bed, there are all kinds of uses there. what on earth would they be doing? >> what you are looking at here,
two things occur along the south waterfront. one, sand that is dredged from the sacramento river is actually put on barges and brought down into the bag at pier 80 and 92, and then they stockpiled the sand and wash it and tip it over to the concrete producers, bodhi, central, and others, rose and jensen, hanson, and the sand is coming down the sacramento river to mix with the aggregate to make concrete. the other thing that happens down in this area of san francisco, we are becoming more and more green at society.
we are taking the asphalt and recycling it, and then we take concrete and put it through -- they will put it through a grinder and grind it to different rock sizes and use that as the aggregate for asphalt or concrete. i am not sure if this is a sand pile for the concrete or if this is a recycle plant. but it is one of them. the city has opened up the large piers that unfortunately are not being used. we don't have the commerce we had in the old days. it has gone to oakland. we don't have the real site on the side of the bay. >> south of market has very unusual conditions, settlement. >> south of market, that particular fixture is at sixth street, which is right in the
heart of this. some of the largest magnitudes of movement during the 1906 earthquake occurred in this area of what is called sullivans march, which it sullivans march, just south of market. not only did it solidify and saddle, but when the ground slopes, and actually flows and moves latterly. some areas of mission creek and sullivan's marsh, that was on the order of 15 feet. it was significant. everything you see south of market was built after the 1906 earthquake.
the longer the time between the 1906 earthquake and the time of construction, the more likely people forgot about all the damage that occurred in 1906. so they were not as sensitive and perhaps cautious as they should be. many of these buildings that we see south of market, while they appear to be in excellent shape and are performing well, have not been either seismically strengthened or had at the subsurface conditions improved, so they are waiting. there could be significant damage during the next major earthquake. >> that reminds me of one of the central questions i wanted to ask you. which is this example relief focuses on the point. that is, what would you
recommend for someone who is considering purchasing a building in the way of a consultation with geotechnical engineers such as yourself, just to get a little heads up on what they might be facing in the lifetime of their ownership of the building? >> i believe it is in a prospective buyer's best interest, if they're buying an existing building and commercial area, that they consult with a structural engineer to make sure the building is structurally adequate and that to consult with a geotechnical engineer. dewitt least point out to them whether the building their purchasing -- to at least point out to them whether the building their purchasing is in a potential hazard zone. it not only has liquefaction,
but it also has a hillside stability issues. while it may be stable sporadically, when the earthquake comes, the slopes may fail. as a minimum, they should retain the services of a geotechnical and structural engineer, and there are well qualified people who come out. i am not just talking about and inspection service, i'm talking that licensed engineer, geotechnical, to look around, look at the maps available, and write an opinion letter as to the adequacy of the structure or the soil. there is another thing to consider when you are buying and building to the east of the van ness ave. a third map.
this map depicts the area of the fire that followed the 1906 earthquake. the blue is where all the buildings were destroyed or badly damaged by the fire. as a result, the ashes from those fires remained on site. in those days, lead paint was used to paint the sides of buildings. it is a known carcinogen. it is a hazardous material. what they did was after the earthquake, they took these mounds of sand, the valencia st. hill, if you go up market street, you know how it starts to climb? imagine that was a much larger hill. they mined 26 million cubic yards of sand out of theire, and
they mixed it with the debris from the fire and they used it to fill basements. so you have sand, ash, pieces of wood, brick. well, the ash is a hazardous material. you come along and used to buy a site at sixth and mission, with the understanding you will demolish the building and build a 50 story building. and you thought you were just going to dig out the sand, the film, and haul it to another site. now you come to find out the material you were going to move at $8 per ton is now going to cost anywhere from $40 to $78 per ton. the has a significant impact -- and has a significant impact on the cost of your project. if you are out on the sunset and
vine a single-family home, just have the geotechnical engineer come out and say it is on earth quicksand, the building will probably settle three-quarters of an inch. have the structural engineer look at the frame and say, you have a serious soft story here. lawrence, expected that. you might want to put in some clips and fasteners and sheer walls to stiffen it out. as you move east, you have not only the soft issues, the structural issues, but you have the hazardous-waste. if you recall as a kid, every intersection had for gas stations? now you cannot find a gas station anywhere? those four gas stations had leaky tanks. now we also have to look at the presence of hydrocarbons in the soil.
>> moving along, the perfect setup for the next question. >> when we went out on our long walk together, we talked about the relationship between the nature of a building, primarily its height, and the soil on which it sits. the slide is a good reminder of that. >> the ground has a natural period. the denser the material, the harder the material, the shorter the period of the ground. bay mud, which is present close to the shoreline, and the green area, the crown has a very long period. when the earthquake comes, there is a link of time that it takes the ground to go from one, slosh back and forth, measured in seconds. the deeper the bay mud deposits, the greater the length of time it takes for the motion to go
one direction and back. so as you approach soft sites, loose sand sites, bay mud sites, the period of the site is long. if you come along and build a tall building, and the ground is going like this, we know that tall buildings, the taller the building, the longer the period, the longer the time it takes to go from one side and back. iand the rule of thumb, every story that you add, it adds a 10th of a second to the period. if you have a 40 story building, you have a building that takes four seconds to go from one side to the other and back. now, that for second building is sitting on 100 feet of bay mud sitting on 100 feet of bay mud that also has a